The objective of the proposed research is to determine the validity of the proposition that the conditioning of contextual stimuli is responsible for the retardation of auto-shaping observed following pretraining with unpredictable food. The research will be conducted in standard operant conditioning chambers for pigeons utilizing food as reinforcement. Previous research has shown that the acquisition of auto-shaping is retarded following pretraining with intermittent, unsignaled food. This retardation effect has been attributed to the prior learning that food is unpredictable. Alternative interpretations based on the establishment of competing behaviors and blocking by background, contextual stimuli have also been advocated. The proposed research will be concerned with the examination of predictions derived from a context blocking analysis. The context blocking analysis predicts that the retardation effect will be context-specific (Experiment 1), that the effects of the context upon acquisition will depend upon the nature of its application (Experiment 2), and that Pavlovian blocking effects occur in the auto-shaping procedure (Experiments 3 and 4).